1. Field of the Invention
This invention generally relates to implantable intralumenal prostheses. More particularly, this invention relates to intralumenal prostheses having stent frames surrounded by biofunctional graft layers and techniques for manufacturing the same.
2. Description of the Background
Stents-grafts have proven to be an effective medical device for minimally invasive treatment of vascular occlusions such as atherosclerosis and restenosis. Stent-grafts are typically shaped as hollow cylindrical structures and constructed of a metal stent with at least one non-metal coating on the stent. The metal stent provides the stent-graft with a structural framework for mechanical support. Non-metal coatings add any number of functions to the stent-graft including delivery of a drug, prevention of thrombi formation in the stent-graft, and reduction of irritation of the vessel wall as compared with the irritation caused by a bare metal stent.
In U.S. Pat. No. 6,165,212, Dereume et al. teach a supportive endolumenal graft. The graft includes a braided tubular support placed over a liner and under a cover. The support is made of metals or alloys, polymers, or ceramics. Exemplary metals and alloys include stainless steel, titanium, tantalum, nitinol, Elgiloy(copyright) and NP35N. Exemplary polymers for making the support include polyurethanes, silicone rubbers, polyether, sulfones, fluoroelastomers, polyimides, polycarbonates, polyethylenes, polylactic acid, polyglycolic acid and polyacrylates. The cover and the liner are made of an elastomeric material, preferably a polyurethane, such as a polycarbonate polyurethane, one commercial example of which is Corethane(copyright) (available from Corvita Corporation of Miami, Fla.).
In U.S. Pat. No. 6,139,573, Sogard et al. teach an elongate radially expandable tubular stent and a polymeric layer covering and conforming to the geometry of the external surface of the stent. A polymeric liner layer and the external polymer layer are laminated together to form a composite structure containing the expandable tubular stent so as to form at least three domains of distinct porosity in the device. The stent may be made from a variety of materials including stainless steel, titanium, platinum, gold, and other bio-compatible metals. Sogard et al. teach that the polymeric layers are made from expanded polytetrafluoroethylene (ePTFE).
In U.S. Pat. No. 6,010,530, Goicoechea teaches a self-expanding stent encapsulated by a skin. The stent is made of a continuous xe2x80x9czig-zagxe2x80x9d nitinol wire wound into a plurality of concentric hoops. The skin is made of an elastomeric polymer, such as Chronoflex (available from PolyMedica Biomaterials Inc., Woburn, Mass.).
In U.S. Pat. No. 5,749,880, Banas et al. teach an encapsulated stent which comprises at least one stent member concentrically interdisposed between at least two tubular ePTFE extrudates, each of the extrudates having a substantially uniaxial fibril microstructure oriented parallel to the longitudinal axis of the stent member.
In U.S. Pat. No. 6,156,064, Chouinard teaches a stent-graft membrane having at least three layers including a structural stent layer, an inside graft layer, and an outside layer. The outside layer is substantially impermeable to fluids. The outside layer is made from a siloxane, polyurethane, polycarbonate urethane, polytetrafluoroethylene (PTFE), ePTFE, or combinations thereof. The graft layer is made from polyethylene tetraphthalate (PET), ePTFE, polycarbonate urethane (PCU), polyurethane, or combinations thereof. The stent filaments may be made of Elgiloy(copyright), Conichrome, Phynox, cobalt-chromium-molybdenum (CoCrMo), titanium alloy, titanium-zirconium-niobium alloy, titanium-aluminum-vanadium alloy (commercially known as TI-6A1-4V), stainless steel, nitinol, platinum, tungsten, tantalum, or combinations thereof.
In U.S. Pat. No. 5,123,917, Lee teaches an expandable intralumenal vascular graft. Lee teaches one embodiment of the graft having an inner layer made from PTFE or a porous polyurethane, an outer layer made of PTFE, Dacron or a proline mesh enclosing the inner layer, and a plurality of spaced scaffold members positioned between the inner layer and the outer layer and made of surgical stainless steel.
In U.S. Pat. No. 5,389,106, Tower teaches a distensible frame and an impermeable deformable membrane interconnecting portions of the frame to form an impermeable exterior wall. The frame is made from a soft platinum wire. The membrane is preferably made from Tactylon(copyright) (available from Tactyl Technologies, Inc. of Vista, Calif.).
In accordance with one aspect of the embodiments of the invention, a stent-graft for biological lumen placement is provided. The stent-graft can include a porous first layer for making contact with tissue of a vessel wall, a non-porous layer substantially encapsulating a frame, and a porous second layer. The non-porous layer can be between the porous first layer and the porous second layer.
In one embodiment of the present invention, the porous first layer can have a void-to-volume ratio of about 40% to about 90%. In another embodiment, the non-porous layer has a void-to-volume ratio of about less than 5%. In yet another embodiment, the porous second layer has a void-to-volume ratio of about 40% to about 90%. In a further embodiment, the pores of the porous first and second layers have an average pore diameter of about 1 micron to about 400 microns.
In accordance with another aspect of the embodiments of the invention, a method for manufacturing a stent-graft is provided. The method of manufacturing can include forming a porous first layer on a mandrel, forming a second layer on the porous first layer, positioning a frame on the second layer, increasing the thickness of the second layer to substantially or completely encapsulate the frame in the second layer, and forming a porous third layer on the second layer.
In one embodiment of the present invention, the act of forming the porous first layer includes applying a composition having a solvent, a polymer dissolved in the solvent, and water-soluble particles to the mandrel and removing the solvent and the water-soluble particles from the polymer. In another embodiment of the present invention, the act of forming the third layer includes applying a composition having a solvent, a polymer dissolved in the solvent, and water-soluble particles to the second layer and removing the solvent and water-soluble particles from the polymer.